Blender VSE cropping a video
That's because your actual screen ratio is 2880/1800 = 1,6. But your desired ratio of 16/9 is 1,7777 (periodic). This doesn't fit together. Blender (by default and there's no way to change that) does scale your input Video to fit into your Render Resolutions Aspect Ratio (I don't mean the Aspect Ratio Pixel size below the Resolution but the actual resulting ratio of your Resolution dividing width by height).
Because your ratio is off you'll get your black bars to the left and right because the rest is fit into the heigth of 1380 pixels which means that your original input should be 3200 pixels wide to fit into your 16:9 format. But as it isn't Blender aligns it horizontally centered and fills the remaining space with black color.

So you'd have to use the Image Crop function of your Strips Strip Input Properties panel in the VSE to precisely set the area that you'd like to cut off.